1. Field of the Invention
The present invention relates to a digital communication system which is resistant against multipath interference.
2. Background Art
In mobile radio communication, circuit design has been considered exceedingly difficult since the radio waves arriving through different paths deteriorate the bit error rate.
In order to solve such a problem, proposed are a double-phase shift keying system (hereinafter referred to as DSK system) and a binary-phase shift keying return-to-zero system (hereinafter referred to as BPSK-RZ system).
Description is now made on the DSK system. As shown in FIG. 3, the DSK system is so devised that it shifts carrier wave phases with respect to binary information symbols .-+.0" and .-+.1" twice by .pi./2 per 1/2 time slot. For example, the carrier wave hases are shifted twice by +.pi./2 with respect to the binary information symbol .-+.1" and twice by -.pi./2 with respect to .-+.0", respectively.
FIG. 3(B) shows the time diagram of phase-shifted DSK signals with respect to binary information symbols "1, 0, 1" in the DSK system performing the aforementioned phase shift operation.
A demodulator employed in the DSK system is shown in FIG. 5. Received signals are divided into two, one is delayed by T/2 (T: time slot length) and the other is not delayed, which are respectively multiplied to be passed through a low-pass filter (LPF), whereby signal e(t) corresponding to the original modulated signal is obtained.
In propagation paths of a mobile radio communication system, the signal from a transmitting point arrives at a receiving point through defraction and reflection by various obstacles. Consideration is now made on two signals shown as D and U waves in FIG. 4, which are transmitted from the same point and are different by .tau. from each other in time received at the receiving point. It has been theoretically confirmed that a signal composed of such two signals shows such error characteristics as shown in FIG. 6(A) when demodulated by the demodulator as shown in FIG. 5.
In FIG. 6, the ordinate shows the bit error rate and the abscissa shows .tau./T (T: time slot width, .tau.: time difference between D and U waves), while symbol Eb indicates signal energy per bit, symbol N.sub.0 noise power per hertz, symbol Pd/Pu average power ratio of the D wave to the U wave and symbol f.sub.D the maximum Doppler frequency. As obvious from the characteristics, the bit error rate is remarkably improved within the range of 0.1 to 0.35 of .tau./T.
The above description has been made on a .pi./2 DSK system which shifts the carrier wave phases with respect to the binary information symbols .-+.1" and .-+.0" twice by .pi./2 per 1/2 time slot, while it has been confirmed that the said description also applies to a .DELTA..theta./2 DSK system for shifting the carrier wave phase by .DELTA..theta./2 (0&lt;.DELTA..theta.&lt;.pi.) per 1/2 time slot.
FIG. 6(B) shows the bit error rate of a .pi./4 DSK system in which the value .pi./2 is replaced by .pi./4, and as obvious from the characteristics shown in FIG. 6(B), the bit error rate is also improved within the range of 0.1 to 0.3 of .tau./T in the .tau./4 DSK system.
The above description has been made on such case where the carrier wave phases are stepwisely shifted in the first and second halves of the time slots as shown in FIGS. 3 and 4, while the same description also applies to the case where the carrier wave phases change smoothly, e.g., to rise in raised cosine curves, for example.
As hereinabove described, the bit error rate is improved within the range of 0.1 to 0.35 or 0.1 to 0.3 of .tau./T in the DSK system, and hence accurate communication is feasible through the use of such a range. However, in a general mobile communication system of a data transmission rate (smaller than several thousand bauds) employing an audio range, the delay time difference .tau. is so small that the value .tau./T is considerably smaller than 0.1, whereby no application can be effected in regions of improved bit error rate where skillful use is made of the characteristics of the DSK system.
Description is now made on the BPSK-RZ system. BPSK-RZ signals are obtained by multiplying normal BPSK signals as shown in FIG. 7(a) by ON-OFF signals as shown in FIG. 7(b), which become .-+.1" in arbitrary T/2 intervals of time slots T. In other words, the BPSK-RZ signals are signals having the same amplitude and phase as that of the normal BPSK signals during the period of the first half or the second half T/2 of each time slot, and exhibiting a substantially zero carrier wave amplitude during the period of the remaining T/2.
The multiple waves of the BPSK-RZ signal are reproduced by a differential detector of FIG. 8. In FIG. 8, (1) is an IN terminal, (2) is a multiplier, (3) is a delay line having delay time of a unit time slot T, (4) is a low pass filter and (5) is an OUT terminal.
The BPSK-RZ signals received in the input terminal 1 are multipath waves formed by overlapping first BPSK-RZ signal waves (hereinafter referred to as D waves) generated by the same digital information and second BPSK-RZ signal waves (hereinafter referred to as U waves) arriving in delay by .tau. from the D waves. FIG. 9 illustrates time relation between the D and U waves. In FIG. 9, symbol T indicates the length of a time slot for transmitting one digital symbol of digital information. Symbol (a) indicates an interval from rise of the D waves to rise of the U waves and symbol (b) indicates an interval from the rise of the U waves to a lapse of T/2 of the D waves while symbol (c) indicates an interval from the lapse of T/2 of the D waves to a lapse of T/2 of the U waves and symbol (d) indicates an interval from the lapse of T/2 of the U waves to a lapse of T of the D waves.
Signals e(t) obtained at the output terminal 5 in the respective intervals are as follows: